Just read an article on an early semi-auto design by a mexican gun designer, Manuel Mondragon.
In 1894 with SIG building the rifles they made 200 guns in .... 5.2 MM .204 caliber! It used a 5.2x68 cartridge with a 6 gram ( 92 grain) bullet. To "capture" the energy of the powder charge, it had a sabot. It looks like a washer, at the base of the bullet, that was the dia. of the cartridge case inside. It must have been pulled off and left inside the case when fired.
And here I thought the .204 Ruger was a new idea!

Dixon's "European Sporting Cartridges" lists such a cartridge on pp 106-07 of vol I with a DWM case number 469A and produced from 1900 to 1907 or so. I'll post pictures of those pages tomorrow.

Added later 3/26: I misread your "5.2x68" as 5.2x58. The reference I wrote of above is to a 58mm case, I have no reference to the longer cartridge.